Do you have any information about the angle of the cone of the signal
The radalts I have worked with are comfortable with a composite attitude angle of around 45°. This would roughly correspond to the half cone angle. I have to say that this is a general observation and is very much dependant on individual units, and their antenna designs and placements.
Would it be right therefore to conclude that radalt would be highly accurate at low altitude over relatively smooth terrain no matter how high the ground speed?
I think that is generally correct, in level flight over a "smooth" reflective surface the ground speed would have little to no effect.
Errors come from a variety of sources, modern radalts are very good at coping with scatter and multi-path effects. They are also largely doppler immune. By design they look for the leading edge of a valid return signal to try and give the closest point and not the slant angle. Therefore if the ground is closer than the slant range to the building then it will still track the ground. There are also filters that prevent sudden large changes of altitude, such as when passing over another aircraft. Accuracy is generally given as a percentage so therefore the lower you are the more accurate the reading. Although there is a lower cutoff due to the limit of technology that gives an absolute measurement accuracy usually around ±3 ft. Because a descent rate is seen directly by the altimeter excessive descent rates can degrade the accuracy as well.